Refusing to assist with a faulty part installed by them!!!

I requested care care click to assist a repair for my car, the changed the thermostat on the 29th Of may 2019, the next day i called them to compliant about a rattling sound on my car right after they have changed this thermostat and they their mechanic said there was nothing wrong with my car, immediately i was experiencing problems with my car not starting as normal and over heating, i took my car in to be checked and i was advised that the thermostat is faulty and not opening. i called the company and they are refusing to assist, they did not even offer to check the problem and blaming me for using another service provider since they failed to assist me. the part should be under warranty since this was installed, they advised that the warranty is void since the other service provider worked on the cooling system and they have messed up the system. The problem was there even before i took the car in to a different service provider because they " could not find a fault" but wanted to charge me for a engine overhaul which i didn't want to pay because the issue arose right after this thermostat was installed.